Hard particle powder for sintered body

ABSTRACT

Provided is a hard particle powder for a sintered body, consisting of: 0.01≤C≤3.5 mass %, 0.5≤Si≤4.0 mass %, 0.1≤Mn≤10.0 mass %, 0.1≤Ni≤35.0 mass %, 0.1≤Cr≤40.0 mass %, 5.0≤Mo≤50.0 mass %, 0.1≤Fe≤30.0 mass %, and 0.01≤REM≤0.5 mass %, with a balance being Co and inevitable impurities.

TECHNICAL FIELD

The present invention relates to a hard particle powder for a sintered body. More specifically, the present invention relates to a hard particle powder to which rare earth metal (REM) is added, which is excellent in terms of powder characteristics or sintering characteristics and which can give a high wear resistance when a sintered body (e.g., a valve seat for car engine) is produced by using the same.

BACKGROUND ART

TRIBALOY (registered trademark) T-400 is well known as a Co-based hard particles that have a high wear-resistance and form a hard phase mainly containing a Mo silicide. A Co-2.5Si-28Mo-8.5Cr-based alloy powder which is an equivalent material of TRIBALOY (registered trademark) T-400 has been frequently used as hard particles that significantly contribute to an improvement of the wear resistance of a valve seat for car engine (hereinafter, simply referred to as “valve seat”) in a car engine to which a high load is applied. Therefore, a number of prior arts have been proposed.

For example, Patent Document 1 discloses a method for manufacturing a wear-resistant sintered member, aiming to disperse a larger amount of a hard layer in a base without impairing wear resistance, strength, or the like. The method includes compression-molding a raw material powder containing a base-forming powder (iron, SUS316, SUS304, SUS310, or SUS430) and a hard layer-forming powder (Co-28Mo-2.5Si-8Cr), and performing sintering. The method is characterized in that 90 mass % or more of the base-forming powder is a fine powder having a maximum particle diameter of 46 μm, and a proportion of the hard layer-forming powder in the raw material powder is from 40 mass % to 70 mass %.

In addition, Patent Document 2 discloses a method for manufacturing a wear-resistant iron-based alloy material for a valve seat, aiming to obtain an iron-based sintered alloy material having excellent wear resistance. The method includes (a) compression-molding an iron-based alloy powder obtained by adding from 0.2 to 3.0 parts by weight of a solid lubricating material powder (sulfide or fluoride) and/or from 0.2 to 5.0 parts by weight of an oxide-stabilizing powder (Y₂O₃, CeO₂, or CaTiO₃, which is an oxide of a rare earth element) to 100 parts by weight of an iron-based alloy powder containing a pure iron powder, an alloy iron powder, a carbon powder, a fine carbide-precipitated steel powder, and a hard particle powder (Cr—Mo—Co-based powder, Ni—Cr—Mo—Co-based powder, etc.); and then (b) performing sintering, thereby obtaining a sintered body.

However, in response to an increase in a load on engine-demanded characteristics, there has been a demand for higher wear resistance for valve seat materials. Therefore, there has been a problem in that the hard particles disclosed in Patent Documents 1, 2, and the like cannot sufficiently satisfy the wear resistance that is demanded for valve seat materials. Furthermore, it is necessary to consider that an attempt to improve the wear resistance that is demanded for valve seat materials is likely to impair powder characteristics (moldability) or sintering characteristics. Therefore, there is a demand for a technique for improving the wear resistance that is demanded for valve seat materials without impairing powder characteristics and sintering characteristics.

Furthermore, in recent years, in order to cope with global-scale social issues such as CO₂ reduction and depletion of petroleum resources, fuel-saving lean-burn combustion techniques such as a direct-injection engine and a homogeneous-charge compression ignition (HCCI) engine, and bioethanol fuel engines using a plant raw material in which no fossil fuel is used are promoted.

A lean-burn combustion engine or an alcohol fuel engine generates a small amount of soot during combustion as compared with a conventional engine. Therefore, there is a concern that, in a low-temperature state after engine ignition, the valve seat is not protected by soot and may be easily worn.

Patent Document 1: JP-A 2007-107034

Patent Document 2: JP-A 2003-193173

SUMMARY

An object that the present invention attempts to achieve is to provide a hard particle powder for a sintered body which is hard particles that are added to a raw material powder of a sintered body and is capable of improving the wear resistance of the sintered body without impairing powder characteristics and sintering characteristics.

In order to achieve the above-described object, a hard particle powder for a sintered body according to the present invention includes:

0.01≤C≤3.5 mass %,

0.5≤Si≤4.0 mass %,

0.1≤Mn≤10.0 mass %,

0.1≤Ni≤50.0 mass %,

0.1≤Cr≤40.0 mass %,

5.0≤Mo≤50.0 mass %,

0.1≤Fe≤30.0 mass %, and

0.01≤REM≤0.5 mass %,

with a balance being Co and inevitable impurities.

The present inventors found that, when a component is optimized in Co-based hard particles including REM, the wear resistance of a sintered body including the hard particles can be improved without impairing powder characteristics and sintering characteristics. This is considered to be because, when an appropriate amount of REM is added to the hard particles, an oxide coating is generated on a surface of the sintered body in a low temperature range of approximately 600° C. and this oxide coating exhibits a lubrication action.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating an outline of a wear tester for a single valve seat.

FIG. 2 is a view for describing a measurement place of a wear amount of a wear test specimen.

FIG. 3 is a view showing the relationships between temperature and weight increase in hard particle powders obtained in Example 2 and Comparative Example 13.

EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described in detail.

1. Hard Particle Powder for Sintered Body

A hard particle powder for a sintered body according to the present invention includes elements as described below, and the balance is Co and inevitable impurities. The kinds of component elements, content ranges thereof, and limitation reasons thereof are as described below.

(1) 0.01≤C≤3.5 Mass %:

In the case where the content of C is excessive, toughness degrades due to the generation of a carbide. Therefore, the content of C needs to be 3.5 mass % or less in the hard particle powder for a sintered body. The content of C is preferably 2.0 mass % or less.

On the other hand, a decrease in the content of C larger than necessary does not make any difference in the effect and does not produce any practical benefit. Therefore, the content of C needs to be 0.01 mass % or more in the hard particle powder for a sintered body. The content of C is preferably 0.5 mass % or more.

(2) 0.5≤Si≤4.0 Mass %:

Si is a component element added aiming to improve hardness due to the generation of a silicide. In the case where the content of Si is too small, the hardness becomes too poor, and the hard particle powder does not function as hard particles. Therefore, the content of Si needs to be 0.5 mass % or more in the hard particle powder for a sintered body. The content of Si is preferably 0.8 mass % or more.

On the other hand, in the case where the content of Si is excessive, the hardness becomes too high. As a result, hard particles crack and drop from a sintered body including the hard particles, and, conversely, the wear amount of the sintered body becomes large. Therefore, the content of Si needs to be 4.0 mass % or less in the hard particle powder for a sintered body. The content of Si is preferably 3.0 mass % or less.

(3) 0.1≤Mn≤10.0 Mass %:

In the case where the content of Mn is too small, the oxide coating is not easily generated on a surface of the powder, leading to a decrease in lubrication property. As a result, wear resistance deteriorates. Therefore, the content of Mn needs to be 0.1 mass % or more in the hard particle powder for a sintered body. The content of Mn is preferably 0.2 mass % or more and more preferably 4.0 mass % or more.

On the other hand, in the case where the content of Mn is excessive, sintering characteristics deteriorate due to an increase in a powder oxidation amount. The content of Mn needs to be 10.0 mass % or less in the hard particle powder for a sintered body. The content of Mn is preferably 7.0 mass % or less.

(4) 0.1≤Ni≤35.0 Mass %:

In the case where the content of Ni is too small, the wear resistance deteriorates due to the degradation of heat resistance. Therefore, the content of Ni needs to be 0.1 mass % or more in the hard particle powder for a sintered body. The content of Ni is preferably 0.3 mass % or more and more preferably 9.0 mass % or more.

On the other hand, in the case where the content of Ni is excessive, the wear resistance deteriorates due to the degradation of the heat resistance. Therefore, the content of Ni needs to be 35.0 mass % or less in the hard particle powder for a sintered body. The content of Ni is preferably 30.0 mass % or less.

(5) 0.1≤Cr≤40.0 Mass %:

Cr is an element added aiming to impart oxidation resistance. In the case where the content of Cr is too small, the wear resistance deteriorates due to the degradation of the oxidation resistance. Therefore, the content of Cr needs to be 0.1 mass % or more in the hard particle powder for a sintered body. The content of Cr is preferably 3.0 mass % or more.

On the other hand, in the case where the content of Cr is excessive, the wear resistance deteriorates due to the degradation of the heat resistance. Therefore, the content of Cr needs to be 40.0 mass % or less in the hard particle powder for a sintered body. The content of Cr is preferably 30.0 mass % or less.

(6) 5.0≤Mo≤50.0 Mass %:

Mo is a component element added aiming to maintain the hardness of powder particles. In the case where the content of Mo is too small, the wear resistance of the sintered body including the hard particle powder becomes insufficient. Therefore, the content of Mo needs to be 5.0 mass % or more in the hard particle powder for a sintered body. The content of Mo is preferably 14.0 mass % or more.

On the other hand, in the case where the content of Mo is excessive, the hardness becomes too high. As a result, the hard particles crack and drop from the sintered body including the hard particle powder, and, conversely, the wear amount of the sintered body becomes large. Therefore, the content of Mo needs to be 50.0 mass % or less in the hard particle powder for a sintered body. The content of Mo is preferably 40.0 mass % or less.

(7) 0.1≤Fe≤30.0 Mass %:

Fe is an element that plays a role of improving the diffusivity of the hard particle powder into an iron powder. In the case where the content of Fe is too small, the hard particles crack and drop from the sintered body including the hard particle powder due to the degradation of the diffusivity into the iron powder. As a result, the wear resistance deteriorates. Therefore, the content of Fe needs to be 0.1 mass % or more in the hard particle powder for a sintered body. The content of Fe is preferably 2.0 mass % or more.

On the other hand, in the case where the content of Fe is excessive, the content of Co decreases. Fe is poorer than Co in terms of heat resistance and wear resistance, and thus, in the case where the content of Fe is excessive, the heat resistance and the wear resistance significantly degrade. Therefore, the content of Fe needs to be 30.0 mass % or less in the hard particle powder for a sintered body. The content of Fe is preferably 20.0 mass % or less.

(8) 0.01≤REM≤0.5 Mass %:

“REM” is defined as a group of elements consisting of Sc, Y and lanthanoid elements (i.e., La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). The hard particle powder of the present invention contains at least one kind of the lanthanoid elements, and a preferred REM is a mischmetal such as an alloy or mixture of La, Ce, Nd, Pr, Sm, and Y, from a view point of industrial inexpensiveness. REM is a component element added to improve the wear resistance of the sintered body including the hard particle powder, without impairing powder characteristics and the sintering characteristics. In the case where the content of REM is too small, REM seldom contributes to the improvement of the wear resistance of the sintered body. Therefore, the content of REM needs to be 0.01 mass % or more in the hard particle powder for a sintered body. The content of REM is preferably 0.05 mass % or more.

On the other hand, in the case where the content of REM is excessive, the sintering characteristics deteriorate due to an increase in the powder oxidation amount, and furthermore, the wear resistance also degrades. Therefore, the content of REM needs to be 0.5 mass % or less in the hard particle powder for a sintered body. The content of REM is preferably 0.3 mass % or less.

2. Method for Manufacturing Sintered Body

The sintered body including the hard particle powder for a sintered body according to the present invention can be manufactured by (a) mixing the hard particle powder for a sintered body according to the present invention, a pure iron powder, and a graphite powder, to obtain a mixed powder, (b) compacting and molding the mixed powder so as to produce a compact body, and (c) sintering the compact body.

2.1. Mixing Step

First, the hard particle powder for a sintered body according to the present invention (hereinafter, also simply referred to as “the hard particle powder”), a pure iron powder, and a graphite powder are mixed together (mixing step). As the blending amounts of the respective components, optimal blending amounts are preferably selected depending on the purpose. In addition, in order to improve moldability, a molding lubricant is preferably added to the raw materials.

In the case where the blending amount of the hard particle powder is too small, the wear resistance of the sintered body degrades. Therefore, the blending amount of the hard particle powder is preferably 5.0 mass % or more in the mixed powder. The blending amount of the hard particle powder is preferably 10.0 mass % or more.

On the other hand, in the case where the blending amount of the hard particle powder is excessive, the sintering characteristics degrade. Therefore, the blending amount of the hard particle powder is preferably 50.0 mass % or less in the mixed powder. The blending amount of the hard particle powder is preferably 30.0 mass % or less.

In the case where the blending amount of the graphite powder is too small, the wear resistance of the sintered body degrades. Therefore, the blending amount of the graphite powder is preferably 0.5 mass % or more in the mixed powder. The blending amount of the graphite powder is preferably 0.8 mass % or more.

On the other hand, in the case where the blending amount of the graphite powder is excessive, the sintering characteristics degrade. Therefore, the blending amount of the graphite powder is preferably 2.0 mass % or less in the mixed powder. The blending amount of the graphite powder is preferably 1.5 mass % or less.

2.2. Compacting and Molding Step

Next, the mixed powder is compacted and molded, thereby obtaining a compact body. Compacting and molding conditions are not particularly limited, and optimal conditions can be selected depending on the purpose. Generally, as a molding pressure increases, a compact density further improves. After the molding, the compact body may be burned in the atmosphere for degreasing.

2.3. Sintering Step

Next, the compact body is sintered (sintering step).

As sintering conditions, optimal conditions are preferably selected depending on a composition of the compact body. Generally, as a sintering temperature increases, a more dense sintered body can be obtained with a heat treatment of a shorter time. On the other hand, if the sintering temperature is too high, there is a problem in that the hard particles excessively diffuse into an iron-based matrix or melt. Although the optimal sintering conditions vary depending on the composition of the compact body, generally, the sintering is preferably performed at from 1,100° C. to 1,300° C. for from 0.5 hours to 3 hours. Furthermore, the sintering is preferably performed in a reducing atmosphere (e.g., in a resolved ammonia atmosphere).

3. Action

In the Co-based hard particles including REM, when the components are optimized, the wear resistance of the sintered body including the hard particles can be improved without impairing the powder characteristics and the sintering characteristics. This is considered to be because, when an appropriate amount of REM is added to the hard particles, an oxide coating is generated on a surface of the sintered body in a low temperature range of approximately 600° C. and this oxide coating exhibits a lubrication action.

EXAMPLES Examples 1 to 30 and Comparative Examples 1 to 44 1. Production of Specimens 1.1 Production of Hard Particle Powders

Raw materials were blended so as to obtain compositions (unit: mass %) shown in Table 1 and Table 2. Raw material mixtures were melted, and hard particle powders were obtained through an atomization method. REM used in the production was a mischmetal that is a mixture of La, Ce, Nd, Pr, Sm, and Y. Table 1 and Table 2 also show sintered densities of the sintered bodies including the hard particle powders and wear amounts of the sintered bodies when a wear resistance test described below was carried out.

TABLE 1 Wear amount Sintered density C Si Mn Ni Cr Mo Co Fe REM (μm) (g/cm³) Ex. 1 1.5 2.5 5 10 10 19 Bal. 15 0.01 17 7.18 Ex. 2 1.5 2.5 5 10 10 19 Bal. 15 0.25 15 7.20 Ex. 3 1.5 2.5 5 10 10 19 Bal. 15 0.50 16 7.21 Ex. 4 0.01 2.5 5 10 10 19 Bal. 15 0.25 18 7.20 Ex. 5 0.75 2.5 5 10 10 19 Bal. 15 0.25 16 7.21 Ex. 6 2.5 2.5 5 10 10 19 Bal. 15 0.25 16 7.18 Ex. 7 3.5 2.5 5 10 10 19 Bal. 15 0.25 19 7.16 Ex. 8 1.5 0.5 5 10 10 19 Bal. 15 0.25 18 7.22 Ex. 9 1.5 1.5 5 10 10 19 Bal. 15 0.25 16 7.21 Ex. 10 1.5 3.5 5 10 10 19 Bal. 15 0.25 16 7.17 Ex. 11 1.5 4.0 5 10 10 19 Bal. 15 0.25 18 7.15 Ex. 12 1.5 2.5 0.1 10 10 19 Bal. 15 0.25 19 7.21 Ex. 13 1.5 2.5 3 10 10 19 Bal. 15 0.25 14 7.19 Ex. 14 1.5 2.5 10 10 10 19 Bal. 15 0.25 18 7.16 Ex. 15 1.5 2.5 5 0.1 10 19 Bal. 15 0.25 12 7.16 Ex. 16 1.5 2.5 5 20 10 19 Bal. 15 0.25 16 7.17 Ex. 17 1.5 2.5 5 27 10 19 Bal. 15 0.25 18 7.19 Ex. 18 1.5 2.5 5 35 10 19 Bal. 15 0.25 19 7.19 Ex. 19 1.5 2.5 5 10 0.1 19 Bal. 15 0.25 14 7.22 Ex. 20 1.5 2.5 5 10 20 19 Bal. 15 0.25 16 7.20 Ex. 21 1.5 2.5 5 10 30 19 Bal. 15 0.25 18 7.17 Ex. 22 1.5 2.5 5 10 40 19 Bal. 15 0.25 19 7.16 Ex. 23 1.5 2.5 5 10 10 5 Bal. 15 0.25 12 7.15 Ex. 24 1.5 2.5 5 10 10 14 Bal. 15 0.25 14 7.17 Ex. 25 1.5 2.5 5 10 10 35 Bal. 15 0.25 18 7.20 Ex. 26 1.5 2.5 5 10 10 50 Bal. 15 0.25 19 7.25 Ex. 27 1.5 2.5 5 10 10 19 Bal. 0.1 0.25 13 7.22 Ex. 28 1.5 2.5 5 10 10 19 Bal. 7 0.25 14 7.19 Ex. 29 1.5 2.5 5 10 10 19 Bal. 23 0.25 18 7.16 Ex. 30 1.5 2.5 5 10 10 19 Bal. 30 0.25 19 7.15

TABLE 2 Wear Sintered amount density C Si Mn Ni Cr Mo Co Fe REM (μm) (g/cm³) Comp. Ex. 1 4.0 2.5 5 10 10 19 Bal. 15 0.25 30 6.97 Comp. Ex. 2 1.5 5 5 10 10 19 Bal. 15 0.25 33 6.92 Comp. Ex. 3 1.5 2.5 0 10 10 19 Bal. 15 0.50 24 7.21 Comp. Ex. 4 1.5 2.5 12 10 10 19 Bal. 15 0.25 36 7.03 Comp. Ex. 5 1.5 2.5 5 0 10 19 Bal. 15 0.25 22 7.16 Comp. Ex. 6 1.5 2.5 5 36 10 19 Bal. 15 0.25 37 7.19 Comp. Ex. 7 1.5 2.5 5 10 0 19 Bal. 15 0.25 23 7.22 Comp. Ex. 8 1.5 2.5 5 10 41 19 Bal. 15 0.25 35 7.14 Comp. Ex. 9 1.5 2.5 5 10 10 4 Bal. 15 0.25 31 7.13 Comp. Ex. 10 1.5 2.5 5 10 10 55 Bal. 15 0.25 39 7.26 Comp. Ex. 11 1.5 2.5 5 10 10 19 Bal. 0 0.25 24 7.22 Comp. Ex. 12 1.5 2.5 5 10 10 19 Bal. 31 0.25 62 7.14 Comp. Ex. 13 1.5 2.5 5 10 10 19 Bal. 15 0 37 7.18 Comp. Ex. 14 0.01 2.5 5 10 10 19 Bal. 15 0 36 7.20 Comp. Ex. 15 3.5 2.5 5 10 10 19 Bal. 15 0 38 7.16 Comp. Ex. 16 1.5 0.5 5 10 10 19 Bal. 15 0 39 7.22 Comp. Ex. 17 1.5 4.0 5 10 10 19 Bal. 15 0 36 7.15 Comp. Ex. 18 1.5 2.5 0.1 10 10 19 Bal. 15 0 38 7.21 Comp. Ex. 19 1.5 2.5 12 10 10 19 Bal. 15 0 36 7.16 Comp. Ex. 20 1.5 2.5 5 0.1 10 19 Bal. 15 0 37 7.16 Comp. Ex. 21 1.5 2.5 5 35 10 19 Bal. 15 0 39 7.19 Comp. Ex. 22 1.5 2.5 5 10 0.1 19 Bal. 15 0 36 7.22 Comp. Ex. 23 1.5 2.5 5 10 40 19 Bal. 15 0 39 7.16 Comp. Ex. 24 1.5 2.5 5 10 10 5 Bal. 15 0 40 7.15 Comp. Ex. 25 1.5 2.5 5 10 10 35 Bal. 15 0 38 7.20 Comp. Ex. 26 1.5 2.5 5 10 10 50 Bal. 15 0 42 7.25 Comp. Ex. 27 1.5 2.5 5 10 10 19 Bal. 0.1 0 33 7.22 Comp. Ex. 28 1.5 2.5 5 10 10 19 Bal. 30 0 40 7.15 Comp. Ex. 29 1.5 2.5 5 10 10 19 Bal. 15 0.7 42 6.98 Comp. Ex. 30 0.01 2.5 5 10 10 19 Bal. 15 0.7 40 7.02 Comp. Ex. 31 3.5 2.5 5 10 10 19 Bal. 15 0.7 41 6.95 Comp. Ex. 32 1.5 0.5 5 10 10 19 Bal. 15 0.7 40 7.02 Comp. Ex. 33 1.5 4.0 5 10 10 19 Bal. 15 0.7 43 6.94 Comp. Ex. 34 1.5 2.5 0.1 10 10 19 Bal. 15 0.7 41 7.01 Comp. Ex. 35 1.5 2.5 12 10 10 19 Bal. 15 0.7 40 6.96 Comp. Ex. 36 1.5 2.5 5 0.1 10 19 Bal. 15 0.7 42 6.95 Comp. Ex. 37 1.5 2.5 5 35 10 19 Bal. 15 0.7 43 6.97 Comp. Ex. 38 1.5 2.5 5 10 0.1 19 Bal. 15 0.7 39 7.02 Comp. Ex. 39 1.5 2.5 5 10 40 19 Bal. 15 0.7 44 6.96 Comp. Ex. 40 1.5 2.5 5 10 10 5 Bal. 15 0.7 38 6.94 Comp. Ex. 41 1.5 2.5 5 10 10 35 Bal. 15 0.7 43 7.00 Comp. Ex. 42 1.5 2.5 5 10 10 50 Bal. 15 0.7 43 7.04 Comp. Ex. 43 1.5 2.5 5 20 20 19 Bal. 0.1 0.7 37 7.02 Comp. Ex. 44 1.5 2.5 5 20 20 19 Bal. 30 0.7 46 6.96

1.2. Production of Sintered Bodies

A mixture consisting of 69.2 mass % of pure iron powder (ASC100.29), 30 mass % of the hard particle powder and 0.8 mass % of a graphite powder (CPB) was prepared. To 100 parts by weight of the mixture was further added 0.5 parts by weight of Zn—St (molding lubricant), followed by mixing.

Next, the raw materials were compression-molded at a molding pressure of 8 t/cm². The shape of the compact body obtained was set to be (a) a disc shape having a diameter of 35 mm and a thickness of 14 mm or (b) a ring shape having an outer diameter of 28 mm, an inner diameter of 20 mm and a thickness of 4 mm

Next, the compact body was degreased at 400° C. for one hour in the atmosphere. Furthermore, the degreased body was sintered at 1,160° C. for one hour in a resolved ammonia atmosphere (N₂+3H₂), thereby obtaining a sintered body.

2. Testing Method 2.1. Powder Characteristics

For the obtained hard particle powders, powder characteristics (particle size distribution, apparent density, flow rate, powder hardness, and oxidation onset temperature) were investigated. Here, (a) the particle size distribution was measured according to Japanese Industrial Standards JIS Z 2510-2004, (b) the apparent density was measured according to Japanese Industrial Standards JIS Z 2504-2012, (c) the flow rate was measured according to Japanese Industrial Standards JIS Z 2502-2012, (d) the powder hardness was measured by using a microhardness measurement instrument, and (e) the oxidation onset temperature was measured by using a thermobalance, respectively.

2.2. Molding Characteristics and Sintering Characteristics

For the produced compact bodies and sintered bodies, molding characteristics and sintering characteristics (compact density, sintered density, chemical components, sintered body hardness, and radial crushing strength) were investigated.

Here, the compact density and the sintered density were measured according to Japanese Industrial Standards JIS Z 2508 and JIS Z2509-2004. The chemical components was obtained through an infrared absorption method. The sintered body hardness (HRA) was measured by using a Rockwell hardness tester. The radial crushing strength was measured by using the ring-shaped sintered body and an Amsler tester.

2.3. Wear Resistance Test of Sintered Body

A wear resistance test was carried out for the sintered body by using a wear tester for a single valve seat, as illustrated in FIG. 1 (hereinafter, also simply referred to as “the wear tester”). Each of the disc-shaped sintered bodies (having a diameter of 35 mm and a thickness of 14 mm) was worked to a valve seat shape and used as individual wear test specimen. In addition, the wear test specimen was set in the wear tester by being pressed into a sheet holder.

The wear tester was driven under testing conditions shown in Table 3. The wear test specimen was worn by a tapping that was input by crank driving while indirectly heating the wear test specimen by heating valves with a gas flame.

TABLE 3 Testing time 10 hours Fuel LPG Contact rate 3,000 times per minute Wear test specimen temperature 300° C. Valve driving Crank shaft Valve rotation rate 10 times per minute Valve face Fe—21Cr—9Mn—4Ni—Co alloy Welding

The shape of the wear test specimen was measured by using a shape measurement instrument before and after the wear test. As illustrated in FIG. 2 (an enlarged view of a portion indicated by an arrow A in FIG. 1), a difference D in a direction perpendicular to the surface of the wear test specimen was obtained and used as a wear amount of the wear test specimen.

3. Results 3.1. Powder Characteristics

Table 4 shows the powder characteristics of the hard particle powders obtained in Examples 1 to 3 and Comparative Examples 9 and 10. FIG. 3 shows relationships between a temperature and weight increase of the hard particle powders obtained in Example 2 and Comparative Example 13. From Table 4 and FIG. 3, the following facts are found. (1) The particle size distributions and the powder characteristics in Examples 1 to 3 were almost the same as those in Comparative Examples 9 and 10. (2) Regarding the particle size distributions in Examples 1 to 3 and Comparative Examples 9 and 10, there were small differences therebetween both in particle size distribution in −100 to +145 mesh and in particle size distribution in −145 to +200 mesh. Therefore, the particle size distributions is considered to result from variation during the manufacturing of the powders. (3) The hardness in Examples 1 to 3 was almost the same as that in Comparative Examples 9 and 10. (4) The oxidation onset temperature was lower in Example 2 than in Comparative Example 13. This is because it became easy for the hard particle powder to oxidize due to the addition of REM.

TABLE 4 Powder characteristics Particle size distribution (mesh, %) Apparent Powder −80/ −100/ −145/ −200/ −250/ density Flow rate hardness +80 +100 +145 +200 +250 +350 −350 (g/cm³) (s/50 g) Hmv Ex. 1 0.0 0.1 8.2 17.1 12.7 23.0 38.9 3.51 20.4 781 Ex. 2 0.0 0.1 8.1 18.0 13.1 21.7 39.0 3.54 20.2 764 Ex. 3 0.0 0.1 8.4 17.5 14.1 20.7 39.2 3.57 20.6 750 Comp. 0.0 0.1 8.2 17.4 13.6 21.2 39.5 3.58 20.8 794 Ex. 9 Comp. 0.0 0.1 8.4 17.3 14.1 21.3 38.8 3.52 20.1 732 Ex. 10

3.2. Molding Characteristics and Sintering Characteristics

Table 5 shows characteristics of the compact bodies and the sintered bodies obtained in Examples 1 to 3 and Comparative Examples 9 and 10. From Table 5, the following facts are found. (1) In Examples 1 to 3 and Comparative Examples 9 and 10, the compositions were different from one another, but almost the same compact density, sintered density, and sintered body hardness were obtained. (2) The radial crushing strength was higher in Examples 1 to 3 than in Comparative Examples 9 and 10. The radial crushing strength is attributed to the sintered body hardness, and thus, when the sintered body hardness is higher, the radial crushing strength also tends to become high.

TABLE 5 Com- Chemical Sintered Radial pact Sintered components body crushing density density (mass %) hardness strength (g/cm³) (g/cm³) C O N (HRA) (MPa) Ex. 1 6.99 7.18 1.35 0.16 0.033 41.9 524 Ex. 2 7.00 7.20 1.33 0.15 0.037 40.8 519 Ex. 3 7.01 7.21 1.38 0.13 0.034 39.5 507 Comp. 6.96 7.18 1.32 0.21 0.035 40.0 498 Ex. 9 Comp. 6.99 7.20 0.83 0.20 0.038 35.4 475 Ex. 10

3.3. Wear Resistance Test

Table 1 and Table 2 show the compositions of the respective hard particle powders, the sintered densities of the sintered bodies for which the hard particle powders were used, and the wear amounts of the sintered bodies in the wear resistance test. From Table 1 and Table 2, the following facts are found. (1) In all of Examples 1 to 30, the wear amounts were less than 20 μm. On the other hand, in all of Comparative Examples 1 to 44, the wear amounts were 20 μm or more. That is, the wear amount became smaller in Examples 1 to 30 than in Comparative Examples 1 to 44.

(2) When Examples 1 to 30 and Comparative Examples 13 to 28 are compared with one another, all of these examples satisfied the preferred component ranges of the present invention except for the presence or absence of REM. Therefore, it was found that, in the component compositions (except for REM) according to Examples 1 to 30, the addition of REM has an effect of improving the wear resistance of the sintered bodies (valve seats). (3) As demonstrated in Comparative Examples 29 to 44, it is found that, when the content of REM is too large, the effect of improving the wear resistance of the sintered body (valve seat) cannot be obtained. From these facts, it is found that the content of REM preferably does not exceed 0.6 mass %. In addition, it is found that the content of REM is preferably 0.5 mass % or less and more preferably 0.25 mass % or less.

(4) In Comparative Example 1, the wear amount was large. This is considered to be because the amount of C was too large and thus, the hardness became high and the hard particle powder was pulverized. (5) In Comparative Example 2, the wear amount was large. This is considered to be because the amount of Si was too large and thus, the hardness became excessively high and the hard particle powder dropped. (6) In Comparative Example 3, the wear amount was large. This is considered to be because the sintered body did not include Mn and thus, no powder oxide film was formed and the lubrication property degraded. (7) In Comparative Example 4, the wear amount was large. This is considered to be because the amount of Mn was large and thus, the powder oxidation amount increased and the sintering characteristics deteriorated. (8) In Comparative Example 5, the wear amount was large. This is considered to be because the sintered body did not include Ni and thus, the heat resistance degraded. (9) In Comparative Example 6, the wear amount was large. This is considered to be because the amount of Ni was too large and thus, conversely, the amount of Co which was a balancing element decreased, and the heat resistance and the wear resistance degraded.

(10) In Comparative Example 7, the wear amount was large. This is considered to be because the sintered body did not include Cr and thus, the heat resistance degraded. (11) In Comparative Example 8, the wear amount was large. This is considered to be because the amount of Cr was too large and thus, conversely, the amount of Co which was a balancing element decreased, and the heat resistance and the wear resistance degraded. (12) In Comparative Example 9, the wear amount was large. This is considered to be because the amount of Mo was too small and thus, the hardness degraded and the wear resistance degraded. (13) In Comparative Example 10, the wear amount was large. This is considered to be because the amount of Mo was too large and thus, the hardness became too high and the hard particle powder dropped. (14) In Comparative Example 11, the wear amount was large. This is considered to be because the sintered body did not include Fe and thus, the diffusivity into the iron powder degraded and the hard particle powder was likely to drop. (15) In Comparative Example 12, the wear amount was large. This is considered to be because the amount of Fe was too large and thus, the heat resistance degraded.

(16) In Comparative Examples 13 to 28, the wear amounts were large. This is considered to be because the sintered bodies did not include REM and thus, oxidation did not occur at low temperatures and the lubrication property on the valve surfaces degraded. (17) In Comparative Examples 29 to 44, the wear amount was large. This is considered to be because the amounts of REM were too large and thus, the powder oxidation amounts increased and the sintering characteristics degraded.

Based on what has been described above, it was found that, in the case where REM is added to a hard particle powder made of a predetermined component system, the wear resistance of a sintered body (valve seat) can be improved while rarely impairing powder characteristics and sintering characteristics, and a sintered body having excellent wear resistance can be obtained.

Hitherto, the embodiment of the present invention has been described in detail, but the present invention is not limited to the above-described embodiment, and a variety of modifications or changes are possible within the scope of the gist of the present invention.

The present application is based on Japanese Patent Application No. 2018-026177 filed on Feb. 16, 2018, the entire content of which is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The hard particle powder for a sintered body according to the present invention can be used as a hard particle powder being included to a variety of sintered bodies that are used as a valve seat, a valve guide, or other mechanical structural components, for the purpose of improving wear resistance. 

What is claimed is:
 1. A hard particle powder for a sintered body, consisting of: 0.01≤C≤3.5 mass %, 0.5≤Si≤4.0 mass %, 0.1≤Mn≤10.0 mass %, 0.1≤Ni≤50.0 mass %, 0.1≤Cr≤40.0 mass %, 5.0≤Mo≤50.0 mass %, 0.1≤Fe≤30.0 mass %, and 0.01≤REM≤0.5 mass %, with a balance being Co and inevitable impurities.
 2. The hard particle powder for a sintered body, according to claim 1, wherein the content of Mn is: 4.0≤Mn≤7.0 mass %.
 3. The hard particle powder for a sintered body, according to claim 1, wherein the content of Ni is: 0.2≤Ni≤30 mass %.
 4. The hard particle powder for a sintered body, according to claim 1, wherein the content of Fe is: 2.0≤Fe≤20 mass %.
 5. A sintered body comprising: a hard particle powder, a pure iron powder, and a graphite powder, wherein the hard particle powder consists of: 0.01≤C≤3.5 mass %, 0.5≤Si≤4.0 mass %, 0.1≤Mn≤10.0 mass %, 0.1≤Ni≤35.0 mass %, 0.1≤Cr≤40.0 mass %, 5.0≤Mo≤50.0 mass %, 0.1≤Fe≤30.0 mass %, and 0.01≤REM≤0.5 mass %, with a balance being Co and inevitable impurities.
 6. The sintered body according to claim 5, wherein the content of Mn is: 4.0≤Mn≤7.0 mass %.
 7. The sintered body according to claim 5, wherein the content of Ni is: 0.2≤Ni≤30 mass %.
 8. The sintered body according to claim 5, wherein the content of Fe is: 2.0≤Fe≤20 mass %. 